Discworld - I just love stories from Terry Pratchet and there are lots of them (40 books).
This is my answer as well. It’s not at all because the world [building] is so compelling – there are better (well, crunchier) books out there for that – but the recurring characters and settings and themes, make this series a total comfort to go back to every few years. It’s like a warm blanket.
Before Disney it used to be Star Wars because it has so much potential for any kind of story. You can go super science fictiony without encountering any kind of fantasy elements. You can have a western type setting, dystopia, magic, drama, comedy, any weird combination of them all.
Earthsea by a country mile.
Roshar, in Brandon Sanderson’s Cosmere books in the Stormlight Archive series. Everything from the magic system and the cultures to the animal life and the weather patterns are all really unique and totally immerse you in the world. Absolute masterclass worldbuilding.
Boring answer, but probably Middle-earth/Arda. Just such a well-crafted world with deep and interesting lore.
Runner up would be Bas-Lag from China Miéville’s books. Very unique, somewhat Lovecraftian steampunk world with many weird inhabitants.